1. Field of the Invention
The present invention relates to coating materials for protecting the surface of diazo-containing materials. More specifically, the invention relates to coating materials which protect the surface of diazo-containing materials from chemical attack and abrasion.
2. Description of Related Art
Diazo-containing materials generally consist of a support carrying a light sensitive diazonium compound. Some diazo-containing materials consist of the diazo material and other layers useful in improving an image to be obtained through the use of the diazo material. Diazo-containing materials are useful for a wide range of applications, including phototool applications in the printed circuit industry, in the production of lithographic plates and for the production of heat-sensitive recording material, among others.
Generally, diazo-containing materials are made by applying a solution of a diazonium compound which may contain other materials such as couplers and optionally a resinous binder to a support. When the support is a plastic film intermediate, anchoring layers may be applied to the film surface to improve adhesion.
When diazo materials comprising a support and a diazonium compound are exposed to a light imagewise, especially ultraviolet (360-440 nm) light, the undecomposed diazonium compound may subsequently be converted into an azo dye thereby providing a recorded image.
Generally, photosensitive diazo materials are developed with water or water vapor, an organic solvent, aqueous or gaseous ammonia or an aqueous solution of a salt after photopolymerization or photodegradation. When moisture, water, various chemicals, foreign particles or fingerprints adhere to the layer of photosensitive diazo material during storage or handling, the utility of the photosensitive material is reduced. Moreover, the surface of the layer of photosensitive diazo material may be easily scratched, locally reducing density, which further decreases utility. In some cases, the diazo-containing material is placed in contact with photoresist or photoimageable soldermask materials which may contain chemicals such as monomers, photoinitiators or solvents which can adversely affect the diazo material. This adverse effect results in either a loss of azo dye density in the image areas or a density build-up in clear areas.
These physical and chemical degradations of photosensitive materials are common problems in the printed circuit industry, particularly in the use of phototools, or photographic masks or stencils bearing a circuit pattern. Such a stencil, which commonly is a diazo film on a polyester base through which a photoresist or soldermask can be exposed, provides an intricate, complex image representing an electrical circuit. The image often consists of many fine lines and junctions spaced closely together. The phototool is used to make printed circuit boards by placing the phototool face down against a photoresist or photoimageable soldermask layer to make a contact print or slightly off contact print by exposing the photoresist or soldermask to high intensity light (360-440 nm) through the phototool. In this way, a single phototool can be used to make up to hundreds of circuit boards.
After initial processing and during use, a phototool must be carefully inspected through a microscope or with automatic optical inspection equipment to insure that there are no breaks in the fine lines of the image which can produce an open circuit in the finished circuit board or unwanted density spots due to particles of dirt or debris which can cause a short circuit. The continued use of the phototool may result in tiny scratches and abrasions on the phototool surface. The photoresists on which the phototool is placed, are usually laminated on copper sheets; small burrs or rough edges of holes bored in the copper sheet can cause scratches as the phototool is transferred from one photoresist to the next. The phototool is frequently wiped with a soft cloth to make sure it is dust and lint free because small particles of dirt can cause scratching as they are wiped across the phototool surface. Because of this general wear and tear on the phototool surface during normal use, the phototool must be frequently reinspected (after 20 to 50 exposures) to insure line continuity and overall phototool integrity. Depending on the size and the intricacy of the phototool, manual microscopic inspections can take up to 2 to 3 hours.
Contact with the photoresist or soldermask also presents various opportunities for the phototool to be damaged chemically by materials remaining in the photoresist or soldermask. In particular, materials remaining in the photoresist or soldermask which are contacted with the phototool can chemically damage the phototool. Further, materials used to clean the phototool or the photoresist or soldermask, such as solvents, may attack the phototool, affecting densities on the surface.
In all cases, overcoats on photographic products must meet exacting requirements with respect to transparency, flexibility, adhesion, and thickness. But, in the case of phototools, even stricter requirements must be met. For example, phototools often require resolutions greater than one line pair/3 mil (76.2 micrometers). In addition, the thickness of the overlay may cause parallax during contact printing resulting in poor wall quality in the final printed circuit.
One method of protecting diazo layers was disclosed in U.S. Pat. No. 3,136,637, issued to Larson, which describes plate or sheet structures comprising a base sheet having coated thereon a stable light-sensitive resin layer such as a light-sensitive diazo resin. Coated over and in contact with the light-sensitive material is an actinic light-transmitting film or layer of a hydrophobic water-insoluble solvent-softenable resin. The processing solutions disclosed remove unexposed diazo-resin/overcoat resin areas. They do not produce azo dye images. Further, these resins show poor permeability to ammonia and water vapor used to produce azo dye images.
U.S. Pat. No. 4,088,492, issued to Pope et. al., describes the incorporation of hydroxyalkyl cellulose ether in a diazo-type sensitizing coating composition or as an overcoat in order to permit rapid release of nitrogen during light exposure and to reduce slippage during roll-to-roll printing of diazo film materials. This coating increases roughness and would increase, rather than decrease, abrasion.
In some cases, in order to prevent damage, users of diazo phototools laminate an adhesive-backed clear polyester layer on top of the diazo layer after processing the diazo image. This protective polyester film is expensive and requires a lamination step which often traps foreign particles and air between the clear polyester film and the diazo surface. The adhesive may also increase background density during successive light exposure to photoresist.
Commonly assigned U.S. Serial No. 494,212, filed Mar. 15, 1990, discloses ammonia and water permeable protective coating compositions for diazo-containing materials of cellulose esters and microcrystalline silica which provide abrasion resistance. These compositions, however, are still susceptible to chemical attack.
Compositions which have abrasion and/or chemical resistance are found in the art for various uses. However, each of these composition formulations lacks the necessary properties to be used for protective coatings for diazo-containing materials.
Mansour et. al., U.S. Pat. No. 3,806,478, relates to protective coatings for paints particularly useful on metal which include an organic liquid medium, a cellulosic precursor and a dispersion polymer of ethylenically unsaturated monomers.
Parsons et. al., U.S. Pat. No. 3,868,338, relates to a thermosetting polyester powder coating composition including polyester, cellulose acetate butyrate, a crosslinking agent, particularly melamine-formaldehyde and a catalyst, particularly p-toluenesulfonic acid.
Quinn et. al., U.S. Pat. No. 4,714,657, discloses thermoplastic articles protected by mar and solvent resistant coatings containing an aminoplast, a polyester polyol, a catalyst, reaction stabilizers and ultraviolet stabilizers.
Wang, U.S. Pat. No. 4,017,556, describes coatings of urethane prepolymer, alkylated melamine/formaldehyde, and an acid catalyst. The resulting coatings have mar resistance, clarity and impact resistance.
GB 2 182 337 relates to surface coverings for floors containing a melamine aminoplast, a vinyl modifier resin, a polyol and an acid catalyst. These coverings are alleged to have resistance to common household stains and improved scratch resistance.
Witman, U.S. Pat. No. 4,983,466, describes floor coverings of cellulose acetate propionate or butyrate, an aminoplast, a polyol, an acid catalyst and chlorinated polyolefin. These coverings provide improved stain and scratch resistance.
Compositions for protecting photographic materials from liquids or solutions such as coffee or ink are disclosed in Helling et. al., U.S. Pat. No. 4,353,980. These compositions include an irradiation-hardening binder, a multifunctional acrylic monomer, diiso- or polyisocyanate and a photoinitiator.
Actinic radiation curable coating compositions which provide abrasion resistant coatings and resistance to attack by chemical and organic solvents are described in Russell, U.S. Pat. No. 4,399,192. The compositions include pentaerythritol tetraacrylate, cellulose ester and a photoinitiator, such as benzoyl peroxide.
None of these compositions provide satisfactory characteristics for protection of diazo-containing materials.
Thus, there is a need in the art for a composition useful for coating diazo-containing materials to protect the diazo-containing material from chemical and physical damage. Particularly, there is a need in the art to provide a composition useful for coating diazo-containing materials to at least partially protect the diazo-containing material from chemical attack. Further, there is a need in the art for an efficient and economical protective coating for diazo-containing materials.